Deenergizing and Energizing the Transformer

It is known that the circuit is energized at the instant that the flow would adopt its maximum negative value (-φmax). At this point, the residual flow is positive, and since the magnetic flux can not be created nor instantaneously extinguished at the moment of energization, the flux will grow from the existing residual flux.

The φt curve has a sinusoidal appearance, regardless of the non-linear characteristics of the circuit. The crest of the φt wave is caused by superposition of the magnetic circuits, and the result is a very high magnetizing current.

As a consequence the residual flow will be added and subtracted from the total flow, increasing or decreasing the inrush current. The time constant of the circuit (L / R) is responsible for the decline of the inrush current.      orc2s boston gear

As already said, after the supply voltage is set to zero, there is still a momentary current flow, but it will decay along the hysteresis loop to the value φR. Since, in a three-phase system, the magnetic fluxes are 120 ° apart, one residual flux will be positive while the other two will be negative, or vice versa.